A number of industrial processes require selectively removing hydrogen from a reaction media. A first example of such a process is the formation of hydrogen gas by the decomposition of ammonia. Ammonia decomposes to form nitrogen and hydrogen. Removing either hydrogen or nitrogen from the system favors the decomposition of ammonia. Selectively removing hydrogen from the mixture depends on several factors, including the temperature and pressure of the reaction mixture. At high temperatures and pressures, the task of separating a particular gas from a mixture of gases requires considerable effort. Furthermore, conventional packed-bed reactors cannot decompose ammonia efficiently. This is primarily because the high concentrations of hydrogen and nitrogen gas involved in the process favor the formation of ammonia.
A second example of an industrial process that may be aided by the selective removal of hydrogen from a mixture of gases is the integrated, gasification combined cycle process (IGCC). The goal of this process is to produce a synthetic gas that is used to power a gas turbine generator. The IGCC process produces a synthetic gas stream that contains trace amounts of ammonia and other impurities, such as hydrogen sulfide (H.sub.2 S). "A Mathematical Model of a Catalytic Membrane Reactor for the Decomposition of NH.sub.3," J. Membrane Science, 78:265-282 (1993). These substances must be removed from the synthetic gas stream. Otherwise, toxic nitrogen oxides (NO.sub.x) are formed when the synthetic gas is burned. The temperatures involved with the process are significantly greater than can be used with conventional gas separation techniques. These temperatures also are significantly greater than the melting or combustion point of most organic membrane materials, which have an upper useful temperature range of about 150.degree.-200.degree. C.
Three commonly used types of membranes include polymers, ceramics and metal membranes, such as palladium or palladium-alloy membranes. Each of these types of membranes have characteristics that prevent their use for separating hydrogen from a mixture of gases at high temperatures and pressures. The low thermal and mechanical strength of polymer membranes makes them unsuitable for reactions involving gases at high pressures and temperatures. The palladium or palladium-alloy membranes are impractical on an industrial scale because of their expense, low hydrogen flux, and because metals deform at high temperatures. As used herein hydrogen flux or hydrogen permeation rate means the amount of hydrogen (molar-flow rate) per unit area of membrane. It is important to note the difference between hydrogen flux and hydrogen permeability. Hydrogen permeability is an intrinsic property of a metal that is used to determine hydrogen permeation rates at a particular hydrogen partial pressure driving force. Ceramic membranes are able to endure high temperature and pressure conditions. However, ceramic membranes allow the mixture of ammonia, hydrogen and nitrogen to flow through the membrane, rather than selectively allowing hydrogen to flow through the membrane. Hence, porous ceramic membranes are insufficiently selective for separating hydrogen from other gases at high temperatures and pressures.
Some of the problems associated with previous membranes have been overcome by combining different types of membranes to increase the overall efficiency of the combined membrane. For instance, metals, such as palladium, have been combined with a porous-glass membrane. More specifically, a thin palladium film of 20 .mu.m or less has been deposited on the outside surface of a porous-glass tube. This overcomes the low thermal stability associated with metals while increasing the rate of hydrogen flux over glass membranes alone. Uemiya et al., "A Palladium/Porous-Glass Composite Membrane for Hydrogen Separation," Chem. Letters, 1687-1690 (1988). However, the glass support does not provide sufficient stability to be used at high temperatures and pressures. Furthermore, Buxbaum et al. produced a membrane comprising a 2 .mu.m palladium film on a niobium disk. However, the Buxbaum et al. procedures are not feasibly applied to extractions of hydrogen at temperatures above about 500.degree. C. At such temperatures, intermetallic-diffusion rates increase, thereby increasing the rate at which the niobium diffuses into the palladium metal. Such membranes eventually become impermeable to hydrogen.
Composite palladium-ceramic membranes also have been made. For instance, Uemiya et al. (Uemiya) described the formation of a composite ceramic-palladium metal membrane that was used for the aromatization of propane. Uemiya et al., "Aromatization of Propane Assisted by Palladium Membrane Reactor," Chem. Letters, 1335-1338 (1990). Uemiya formed the composite membrane by depositing a palladium metal layer on the outside surface of a ceramic tube. Uemiya taught a palladium metal layer having a thickness of 8.6 .mu.m. Moreover, Uemiya specifically stated that the promoting effect of the membrane increased with increasing palladium thickness. However, Uemiya teaches nothing about (1) selectively removing hydrogen from a mixture of gases at high temperatures and pressures, or (2) selectively separating hydrogen from a mixture of gases to promote the decomposition of ammonia. The process described by Uemiya used a transmembrane pressure of about 1 atmosphere. Palladium film thicknesses of less than about 10 .mu.m have defects when the palladium film is deposited using an electroless deposition process such as taught by Uemiya. These defects reduce the effectiveness for selectively removing hydrogen from a mixture of gases at transmembrane pressure differences of greater than about 1,000 kPa. Thus, the Uemiya membrane would not work for removing hydrogen selectively from a mixture of gases at high temperatures and transmembrane pressures such as would be encountered in the IGCC process or a process whereby hydrogen gas is formed by decomposing ammonia.
Hence, a need exists for a membrane that selectively removes hydrogen from a mixture of gases at high temperatures and pressures via a semipermeable membrane.